The objective is to investigate histochemically and ultrastructurally the motor innervation of normal and reinnervated muscle spindles in the cat. Quantitatively precise figures for the detailed pattern of the distribution of diverse motor nerve terminals to the three types of intrafusal muscle fiber in a spindle will be obtained. The study will help to understand how the function of the mammalian muscle spindle is modulated by motor nerves, and what role, if any motorneurons play in the regulation of the structural properties of intrafusal muscle fibers.